Pectinases in leaf degradation by aquatic Hyphomycetes

Abstract

Packs of oak and alder leaves were submerged in late autumn in the River Bourne, a moderately eutrophic stream in Surrey so that the colonization pattern of aquatic Hyphomycetes on the leaves could be quantified as the leaves were degraded. The physico-chemical of the water was monitored over the experimental period and the inoculum available for leaf colonization was measured by filter counts of conidia in the stream. Colonization of the leaves by pectolytic bacteria was also measured. There was one series of oak packs and three of alder submerged a fortnight apart. Total spore counts/g dry wt of leaf rose to a peak for all series, followed by a decline. The time to peak colonization was slower in oak than in alder, and in the alder series was shown to depend on the level of inoculum in the stream. Pectolytic bacteria counts followed the pattern of total spore counts, suggesting the exploitation of the same substrates by bacteria and fungi. Alder I was skeletonized in 10wks., Alders II and III in 12wks. and oak in 25wks.5g leaf packs were regarded as having a 'unit-community'of microbes. There was an association of 4 dominant species of aquatic Hyphomycetes on all leaf packs, with about 10 occasional species. Colonization involves the selection of the dominant species from the available inoculum. Within the association there is evidence of competitive interactions and a degree of substrate specificity. The species equilibrium is 14 for all series. Species numbers/pack are initially low, rise to a peak, then decline. A quantitive analysis was made of species lists from the literature, to place the species list from the Bourne in a general context. Reciprocal averaging provided satisfactory results. It emerges that three factors influence the species of aquatic Hyphomycetes found in a stream: the geographical location; the physico-chemical quality of the water, and the plant species providing allochthonous litter. 7 aquatic Hyphomycete isolates from the stream could elaborate both polygalacturonases and pectin transeliminases. Some would grow at pH5 and 7; others only at pH7. PG's were produced constitutively and induced production of PTE's. Tricladium spendens and Articulospora tetracladia both elaborated 3 PG isoenzymes each of which was purified and characterized. Tetrachaetum elegans produced an exo-PTE and a pectin methylesterase. Mycocentrospora angulata elaborated an endo-PTE and a PME. The PTE's and PME's were partially purified and characterized.All four species macerated strips of alder leaf completely within 9 - 12 days, utilizing PTE's mainly. Egglishaw's observation that plant degradation is most rapid in streams of high calcium concentration is likely to be due to the stimulating effect of calcium ions on micro-bial PGTE and PTE enzymes. Their activity increases with increasing Ca2+ concentration.